1. Field of the Invention
The present invention relates to a thermoelectric power generator and more particularly to a thermoelectric power generator capable of generating power not only from solar heat and geothermal heat but also from a heat source of medium or low temperature which has been impossible to be utilized by conventional arts, with high efficiency of thermoelectric conversion.
2. Description of the Related Art
At the present time, heat energy is converted into electric power mainly by a heat engine in which the process is: heat energyxe2x86x92high pressure steamxe2x86x92turbinexe2x86x92generatorxe2x86x92electric power.
This method of converting heat energy into electric power have greatly contributed as power sources for supporting people""s life in society, but there is a problem that it accompanies a large waste heat with its thermal efficiency of 45% at the best.
To combine a gas turbine to this method has already been attempted to good purpose, and it is applicable to thermal power generation using fossil fuels but not applicable to nuclear power generation.
Power generation by fuel cells seems like a promising one from a point of view of largely improving thermal efficiency, however at present the usable fuel is limited to hydrogen which is comparatively expensive and there still remain problems.
On the other hand, the conversion of heat energy into electric power using devices based on Seebeck effect is already established, but its thermal efficiency is 20% at the best and not in the state of general use on a large scale.
Electric power will continue without doubt to be necessary as an important energy to support people""s life in society in the future. A requisite for the process of obtaining electric power from heat energy is to attain thermal efficiency as high as possible now that global environment crisis is strongly acknowledged.
However, there is a theoretical upper limit which can not be exceeded in thermal efficiency of each process of converting heat energy into electric power, and in any of the processes its thermal efficiency has reached near the upper limit by continued effort. Therefore, a leap in the concept of the method of thermoelectric conversion itself is necessary to get a quantum leap in thermal efficiency.
The inventors have investigated the operating mechanism of already-existing solar cells and devices utilizing Seebeck effect and worked toward development of a thermoelectric conversion device which operates at room temperature and moreover without large temperature difference in the device.
Two inventions made heretofore were applied for patent; the first one is disclosed Japanese Unexamined Patent Publication 6-151978 and the second is disclosed Japanese Unexamined Patent Publication 8-306964.
Here, the idea and invention obtained through a series of studies made previously will be described, in which mention will also be made of the significance of the present invention.
The basic configuration of the thermoelectric power generator according to the present invention is as follows: 
Basic operation is as follows:
1) Electrons are thermally excited from valence band to conduction band in the semiconductor.
2) When an appropriate electric field exists in the semiconductor, the thermally excited electrons gather in region (C) of conduction band while holes gather in the region (A) of valence band. This is charge separation by the built-in electric field.
3) However, in the state cited above, the electrons and holes are in the state of thermal equilibrium, therefore the Fermi level is the same between region (A) and region (C).
4) If the electrons gathering in region (C) of conduction band can rise to higher Fermi level than that in region (A), then the electrons obtain the capability of flowing in an external circuit from cathode to anode and working electrically at the load.
Thus, the heat for thermal excitation of electron from valence band to conduction band in the semiconductor is converted into electric power. A temperature difference between both electrodes is not necessary, which is different from the case of Seebeck effect. Therefore, as the heat energy flowed into the body does not flow out to anywhere but converted into electric power, thermal efficiency would be 100%. This is the idea of thermoelectric conversion that occurred to the inventors. The inventors have made repeated studies to realize the idea, and made several key inventions cited below.
The band gap of semiconductor is desirable to be equal or under 1 eV in order to induce the thermal excitation of electrons in between the energy bands at room temperature or a little higher temperature, which is well known. The condition for building in an appropriate internal electric field to separate the carriers excited in on the energy bands is also publicly known. That is, in the configuration of an device shown below, 
the condition for building in appropriate internal electric field to gather holes to region (A) of valence band and electrons to region (C) of conduction band is:
with n-type semiconductor;
xcex94xcfx86A=xcfx86ANxe2x88x92xcfx86n greater than (Egxe2x88x920.2)/qxe2x80x83xe2x80x83[1]
xcex94xcfx86C=xcfx86nxe2x88x92xcfx86CA greater than 0xe2x80x83xe2x80x83[2]
with p-type semiconductor;
xcex94xcfx86A=xcfx86ANxe2x88x92xcfx86p greater than 0xe2x80x83xe2x80x83[3]
xcex94xcfx86C=xcfx86pxe2x88x92xcfx86CA greater than (Egxe2x88x920.2)/qxe2x80x83xe2x80x83[4]
where symbols denote
xcfx86 work function (v)
Eg band gap (eV)
q charge of an electron
A position (A)
C position (C)
n n-type semiconductor
p p-type semiconductor
AN anode
CA cathode.
The present invention establishes a Fermi level difference between region (A) and (C) by increasing the minority carrier density in either plane of a semiconductor beyond the thermal equilibrium state by external action.
The configuration of a device the inventors proposed as a means for realizing the idea mentioned above is that, tellurium (Te) is used as a semiconductor, copper (Cu) as an anode, aluminum (Al) as a cathode, The anode and cathode each is brought into close contact with the solid tellurium, and further glycerol is contacted to the cathode side. Properties of matter are as follows:
These values of properties suffice the required conditions [3] and [4]. Further, electrons liberated by the reaction of Al with glycerol are injected into tellurium (Te) at the cathode.
According to the idea of the inventors, the electron, which is minority carrier in tellurium (Te), externally injected with high electrochemical potential exceed the equilibrium state in both energy level and density, and would raise the Fermi level in region (C). The idea was verified by the experiments and the inventors disclosed it in Japanese Unexamined Patent Publication 6-151978. Though the invention enabled the device for thermoelectric conversion, further increase of power output was required.
Further, crystalline semiconductor such as tellurium is not suitable for producing a sheet-like semiconductor of large area. Producing a semiconductor in a sheet of large area is necessary for mass production of thermoelectric power generator, and a semiconductor suitable for this purpose should be selected.
The inventors hit upon an idea of using sulfide semiconductor. This is based on the characteristic that sulfide semiconductor is of ionic bonding and is functioning well by a comparatively easy production method. An idea of producing a sheet of large area is that the fine particles of sulfide semiconductor obtained by liquid phase reaction at normal temperature are shaped into a solid matter and hardened using an appropriate supporting material and binder.
It is necessary that the sulfide semiconductor is in a state of low hydration, and the fact that it contains water achieves an important role as mentioned later. The electron affinity x of sulfide semiconductor was assumed to be 3.6xcx9c3.8 V, and further the following materials and the like which were semiconductors having band gap Eg of equal or smaller than 1 eV we re selected as constituent of the device:
Cu2S(p-type, assumed Eg=0.6eV)
FeS(n-type, assumed Eg=0.7eV)
The power output of the device mentioned before using tellurium as semiconductor is small because of small difference of Fermi level between region (A) and (C).
Thus, an idea occurred to th e inventors was: electrochemical reaction having low Fermi level is allowed to exist steadily in region (A); on the other hand electrochemical reaction having high Fermi level is allowed to exist steadily in region (C); the difference of both Fermi levels is applied to the semiconductor as minus bias voltage; and a large difference in Fermi level is established between region (A) and (C).
Here exist two preconditions. The first is that the reaction potential generated in region (A) and that generated in region (C) should be linked. To realize the linkage, the semiconductor layer existing between region (A) and (C) is required to be in the state of a salt bridge. A semiconductor in the hydrated state is necessary for salt bridge formation and is attained only by the method, as mentioned above, in which fine particles of semiconductor are formed to a solid body while containing water. A crystalline semiconductor can not meet this requirement.
The second is that excessive diode current should not flow in the state when the minus bias is applied. This is attained by allowing sufficiently high schottkey barrier to exist in region (A), or allowing potential barrier due to p-n junction to exist in the central region between region (A) and (C).
In the thermoelectric power generator prepared by this invention, the potential barrier existing internally for separating the thermally exited carriers in between energy bands contributes advantageously to restrain the diode current.
The inventors began by selecting a redox reaction system composed of an aqueous solution of {Cu+(NH3)2xcx9cCu2+(NH3)2+n(n=0, 1, 2)}.
The reaction potential of this reaction group exists in the favorable region as follows:
E0=0.06V vs NHE (Normal hydrogen electrode) (where n=2)
"psgr"A=xe2x88x924.49V vs Vacuum
Further, the reaction system has charge transport ability as a characteristic of redox reaction, which is also a preferable feature.
Then, the inventors thought of allowing the reaction between S2xe2x88x92 constituting semiconductor and metal cathode having reaction affinity with S2xe2x88x92 to exist in the region (C) in equilibrium state.
Cathode material+S2xe2x88x92Sulfide+2exe2x88x92
In the above reaction equation, rightward progress consuming cathode material and S2xe2x88x92 should be suppressed.
This is realized by eliminating strange electrophilic centers which force rightward progress via irreversible consumption of electrons liberated in the reaction. A redox reaction comprising an electrophilic center does not consume electrons irreversibly so long as both reactions, reduction and oxidation are in dynamically balanced state.
The reaction potential difference obtained by linking electrochemical reaction in region (A) and that in region (C) is sufficiently large as shown in Table 1.
To provide a redox reaction system to an anode side is well known in the art of wet-type solar cell. However, the finding that a Fermi level difference can be established by allowing reaction potential of electrochemical reaction to exist at both the anode side and the cathode side and by allowing the difference between both reaction potentials to impress on a semiconductor layer as a minus bias voltage is a new discovery obtained by the inventors, by which the possibility of utilizing the thermal excitation phenomenon for a thermoelectric conversion was opened. The inventors have applied for patent with a series of the inventions mentioned above as disclosed in Japanese Unexamined Patent Publication 8-306964.
However, there remained a problem that the corrosion of cathode material should be suppressed in the method according to Japanese Unexamined Patent Publication 8-306964. The corrosion is caused by the fact that the redox reaction liquid existing in the anode region osmoses gradually into the semiconductor layer and intrudes into the cathode region where it reacts with the cathode material. As a natural result, the damage of cathode deteriorates the durability of the device.
To cope with this problem, the inventors tried at first to lower the liquid permeability of the semiconductor layer as low as possible but did not succeed in achieving at the same time two mutually contradictory requirement, i.e. to link the reaction potentials generated at both planes of the semiconductor layer and to decrease the liquid permeability of the layer.
As a next approach, the inventors hit upon an idea in that the aqueous solution of {Cu+(NH3)2xcx9cCu2+(NH3) 2+n(n=0, 1, 2)} is occluded in a suitable adsorbent, a necessary amount of binder is added, and further a sufficient amount of ammonium salt is added for the reason mentioned later to solidify and harden the reaction liquid for depriving it of fluidity in order that no reaction liquid may intrude into the cathode range.
The inventors found that active carbon showed a superior performance among a variety of existent adsorbents and succeeded in solidifying redox reaction liquid.
Further, the electrolyte aqueous solution room was eliminated from the cathode region in correspondence with the solidification of the anode reaction liquid, because if electrolyte aqueous solution remains in the cathode region the liquid intrudes into the anode region to allow the elution of the redox reaction system solidified resulting in the loss of effect of the solidification.
The inventors thus succeeded in generating large continuous power output by the thermoelectric power generator in which a solidified redox reaction system is provided in the region of an anode and further the region of a cathode is reduced to the semi-dried state where the cathode contacts a semiconductor. Moreover, this solid state construction is simple, free from trouble such as leakage of liquid, and suitable for commercialization. The invention cited above is the skeleton of the present application.